1. Field of the Invention
This invention relates to the preservation of moist plant-derived raw materials. More particularly the invention relates to the preservation of wood substance, brightness and inherent tall oil in wood chips during stockpiling.
2. Description of the Prior Art
Outside storage of wood chips, introduced in the early 1950's, resulted from the need to stockpile chips produced from residues of sawmills and veneer plants. As the economic advantages of handling wood as a bulk material become apparent, many mills began chipping and stockpiling chips as an alternative to log storage.
With increased use of this method for handling and storage of wood, a serious disadvantage became apparent. Stockpiled wood chips characteristically lose wood substance, brightness, and tall oil during storage.
One principal difference between log and chip storage is that a significant amount of heat is evolved in piled chips but not in piled logs. Center temperatures in chip piles frequently reach and remain in the vicinity of 150.degree. F. thus resulting in deterioration of the material.
Heat production in chip piles is primarily caused by the respiration of a rapidly multiplying population of bacteria, the enzymatic respiration of any living wood cells, respiration of fungi and direct chemical oxidation. Prevention of the heat release from bacteria, fungi and living wood cells would stop the initial temperature rise and prevent the direct chemical reactions from releasing any significant amount of heat.
A chemical treatment used to prevent chip deterioration and resulting loss in tall oil should reduce the initial heating by inhibiting respiration of bacteria and fungi and additionally the respiration of any living wood cells that are present. A treatment that kills the bacteria, fungi and the living wood cells would be highly effective. The chemical treatment should also meet the following criteria. It should be effective for a reasonable length of time and its cost should be less than the losses incurred from chip deterioration. It should be compatible with the pulping process, should not cause pollution, and obviously should not be hazardous to personnel who handle and apply it. When used with foodstuffs such as wheat or corn, the treatment must meet the additional requirement of being nontoxic to humans and animals.
One method that has been found successful in reducing wood substance losses in stored wood chips, by limiting the effect of wood-destroying fungi on the chips, can be found in U.S. Pat. No. 3,646,196. This method inherently produces a large reduction in wood brightness and has little favorable effect on tall oil retention. A subsequent development, disclosed in application Ser. No. 431,762 filed Jan. 8, 1974, now abandoned, uses sodium N-methyldithiocarbamate. In addition to reducing wood substance losses this treatment retards brightness losses. The present invention, however, works effectively against all three major effects of deterioration, i.e., wood substance, brightness and tall oil losses. With the cost of tall oil as high as it is today, the preservation of this product during wood chip storage is vital.
Sodium bisulfite has been used as a preservative with wood and is effective for a limited period (about one month according to the inventor's tests). Phenolic compounds have also been used to preserve various cellulosic materials but are ineffective in reducing heat buildup in chip piles. The present invention has combined these two chemicals into a treatment which far surpasses in effectiveness either of these components individually or what one skilled in the art would have expected their combined effectiveness to be.